EXPERIMENTAL STUDY ON NH3/H2/AIR COMBUSTION IN SPARK-IGNITION ENGINE CONDITIONS

International audienceThe mitigation of climate change implies the increasing use of variable renewable energy sources. Energy storage and transport solutions will contribute to ensure the stability, reliability and flexibility of the energy systems. Ammonia is a well-known chemical of formula NH3 and, amongst other electrofuels, a promising energy carrier and carbon-free combustible fuel. There-fore, it is of significant interest to study ammonia combustion systems. The present investiga-tion focusses on premixed ammonia/hydrogen/air combustion to assess stability ranges, perfor-mance and pollutant emissions by means of a systematic parametric study, in the purpose of optimization in the case of a current spark-ignition engine. Gaseous ammonia blends with a wide range of hydrogen fuel fractions and equivalence ratio were tested at two different loads. Results show a power output and indicated efficiency benefit of low H2 enrichment for slightly rich and slightly lean mixtures, respectively. Hydrogen is therefore mainly an ignition promoter, rather than a global combustion promoter assumedly due to high thermal losses. A small amount of H2, along with supercharged operation greatly improves the performances of the engine and its stability, thus rendering NH3 a very suitable fuel for SI-engines in case of in-situ H2 gener-ation. Hydrogen also mitigates unburned NH3 emissions, yet not sufficiently but those could be combined with the evenly elevated NOx emissions in dedicated selective catalytic reduction systems

Uncertainty in measuring laminar burning velocity from expanding methane-air flames at low pressures

International audienceThe experimental determination of laminar burning velocity remains essential to evaluate the combustion potential of any fuels but also to validate kinetic mechanisms. Recently, researchers are making great efforts to improve the accuracy of the different setups and techniques to determine this parameter. This work proposes an attempt to summarize the different factors contributing to the uncertainty of the expanding spherical flame method. In particular, the validity of two hypothesis (adiabatic flame propagation and thin flame front) is discussed in the case of stoichiometric methane-air flames in low-pressure environment (from 0.2 to 2 bar). Last, the effect of spark electrode diameters was also considered (0.2, 0.5 and 1 mm)

Turbulent partially cracked ammonia/air premixed spherical flames

The combustion of ammonia requires, for most energy conversion systems, a combustion promoter such as hydrogen to guarantee the start-up, stability and combustion efficiency. Partially cracked ammonia (PCA) can provide sufficient hydrogen concentrations to enhance the burning velocity in comparison with pure ammonia. However, little work exists on the use of PCA blends operating under relevant turbulent conditions. To that end the outwardly propagating spherical flame configuration was employed to determine the laminar and turbulent flame propagation characteristics of PCA (NH3/(H2+N2)) and corresponding binary (NH3/H2) mixtures across various turbulent combustion regimes. First, PCA and ammonia-hydrogen blends exhibit similar flame propagation rates under various turbulent intensities, even for the laminar case. The highest turbulent burning velocity was observed at leanest conditions, as opposed to laminar flames which exhibited highest flame speed at conditions above stoichiometry. Under rich conditions, no substantial flame enhancement due to turbulence was measured irrespective of the hydrogen content. This lack of flame enhancement under turbulent conditions is attributed to the effect of preferential diffusion with good agreement observed with trends in measured Markstein numbers. The normalized turbulent flame speed is dominated by the enhanced molecular diffusivity afforded by the presence of hydrogen up to 15 % enrichment, prior to decreasing upon further hydrogen addition under lean and stoichiometric conditions. This ‘bending’ phenomenon may be the contribution of several factors including; the transitioning between combustion regimes associated with low Damköhler numbers (Da) and flame thickening; merging of flamelets due to the presence of ammonia enhancing wrinkling; and combined changes in laminar burning velocity and preferential diffusional behavior. Furthermore, good agreement for turbulent flame speed is observed with a correlation that includes the influence of turbulent stretch (Ka) and non-equidiffusion (Le), with the agreement reducing with decreasing chemical to turbulent time scale ratios (Da << 1)

Combustion, Performance and Emission Analysis of an Oxygen-Controlling Downsized SI Engine

In the present study, experiments were carried out in a single-cylinder downsized SI engine with different rates of oxygen (15% to 27% by volume in the total mixture of intake gases except fuel) and equivalence ratios (from 0.45 to 1). Therefore, the oxygen volume fraction is due to oxygen enrichment or nitrogen dilution. The study of the impact of controlling the oxygen concentration on the combustion characteristics and emissions was performed at 1 400 rpm, at several loads (Indicated Mean Effective Pressure (IMEP) from 400 to 1 000 kPa). For each operation point, the spark advance and the intake pressure were adjusted simultaneously in order to maintain the load and obtain a minimum value of the indicated Specific Fuel Consumption (SFC). The effect of the oxygen concentration on the engine combustion characteristics was simulated by using the commercial software AMESim, with the combustion model developed by IFP Energies nouvelles, and an adapted algorithm was used to avoid residual gas calibration. By implementing a correlation for the laminar burning velocity, the in-cylinder pressures were perfectly predicted with a maximum pressure relative error of less than 2% for almost all the operating points. The classification of engine combustion according to the Peters-Borghi diagram, gives a deeper insight into the interaction between turbulence and the flame front

Laminar burning velocities of premixed nitromethane/air flames: an experimental and kinetic modeling study

International audienceDue to its high lubricity, nitromethane is a fuel regularly used in model engine or more generally in race engine. The objective of this study is to improve our knowledge and understanding of the combustion of nitromethane for better evaluating its potential as fuel for automotive spark-ignition engines. To achieve this goal, unstretched laminar burning velocities of nitromethane-air mixtures were measured using spherical propagation methodology at 423 K over a pressure range from 0.5 to 3 bar and equivalence ratios from 0.5 to 1.3. The data indicated a typical adverse effect of pressure on laminar burning velocities. Based on the work done by Zhang et al., Proc. Combust. Inst., 33 (2011) 407-414, a modified detailed kinetic model including 88 species and 701 reactions was proposed. Comparisons between experimental and simulated un-stretched laminar flame speed were made and showed good agreement. The new kinetic mechanism was also used to successfully simulate published experiments and rationalize the unusual occurrence of maximum flame speed in the fuel-lean region

Engine performances and emissions of second-generation biofuels in spark ignition engines: The case of methyl and ethyl valerates

As an alternative to second generation ethanol, valeric esters can be produced from lignocellulose through levulinic acid. While some data on these fuels are available, only few experiments have been performed to analyze their combustion characteristics under engine conditions. Using a traditional spark ignition engine converted to mono-cylinder operation, we have investigated the engine performances and emissions of methyl and ethyl valerates. This paper compares the experimental results for pure valeric esters and for blends of 20% of esters in PRF95, with PRF95 as the reference fuel. The esters propagate faster than PRF95 which requires a slight change of ignition timing to optimise the work output. However, both the performances and the emissions are not significantly changed compared to the reference. Accordingly, methyl and ethyl valerate represent very good alternatives as biofuels for SI engines. Future studies will focus on testing these esters in real application engines and performing endurance tests. Copyright © 2013 SAE International

Experimental and numerical analysis of nitric oxide effect on the ignition of iso-octane in a single cylinder HCCI engine

Small concentrations of very reactive chemical species such as nitric oxide (NO) have a very important impact on the onset of combustion. In engine applications, this may lead to irregular combustion phenomena in spark ignition engines or contribute to the control of the homogeneous charge compression ignition (HCCI) engines. To numerically analyze and predict the effect of these species, detailed chemical mechanisms are required. However, including these mechanisms in computational fluid dynamics (CFD) simulations often results in prohibitive computational cost. Using a single-cylinder HCCI engine test bench, we have analysed the effect of initial NO concentrations ranging from 0 to 500. ppm on the ignition of iso-octane. We have also investigated whether the tabulation of dynamic adaptive chemistry (TDAC) method, that significantly reduces the CPU time associated with using detailed chemistry in CFD simulations, could capture this effect. This paper first presents the experimental setup and the validation data. Then it compares these experimental data with the CFD simulation results using a detailed mechanism involving more than 1000 species and 4500 reactions. The significant effect of NO on iso-octane ignition is efficiently captured over the whole range of NO concentration with a speed-up factor of up to 1500 (compared to simulations without TDAC). This work further demonstrates that TDAC represents a very efficient tool to include detailed kinetic mechanisms able to describe complex phenomena, such as the kinetic effect of small concentrations of reactive species, on ignition in a HCCI engine. © 2013 The Combustion Institute

Combustion and emissions characteristics of valeric biofuels in a compression ignition engine

New-generation biofuels are mainly produced from nonfood crops or waste. Although second-generation ethanol is one of the main options, valeric esters can also be produced from lignocellulose through levulinic acid. However, only few experimental results are available to characterize their combustion behavior. Using a traditional compression ignition (CI) engine converted to monocylinder operation, the engine performances and emissions of butyl and pentyl valerate (BV and PenV, respectively) were investigated. This paper analyses the experimental results for blends of 20%vol of esters in diesel fuel, taking diesel fuel as the reference fuel. The BV and PenV have a smaller cetane number and consequently the ignition delay of the blends is slightly longer. However, engine performances and emissions are not significantly modified by adding 20%vol of esters to diesel fuel. The BV and PenV then represent very good alternative biofuels for CI engines. © 2014 American Society of Civil Engineers